Method of electroplating bright white gold alloy coatings

ABSTRACT

An aqueous electrolytic bath for use in depositing bright gold plating is provided, which includes a gold cyanide as the gold source, an alkali metal nickel cyanide, an alkali metal zinc cyanide, which nickel and zinc codeposit with the gold, an alkali metal cyanide, at least one conducting salt and an alkaline compound to maintain the electrolytic bath at a pH within the range of from about 9 to about 13. In addition, a method of electroplating gold alloy employing the above gold plating solution and gold-plated substrates or articles produced thereby are also provided. The electroplate produced employing the above electrolytic bath will comprise from about 75 to about 95% gold, from about 5 to about 15% nickel and from about 1 to about 8% zinc and will have a white color characterized by stain and corrosion resistance and by a bright and aesthetically appealing appearance.

United States Patent [1 1 Greenspan Oct. 28, 1975 METHOD OF ELECTROPLATING BRIGHT WHITE GOLD ALLOY COATINGS [75] Inventor: Lawrence Greenspan, New York,

[73] Assignee: Engelhard Minerals & Chemicals Corporation, Murray Hill, NJ.

[22] Filed: Dec. 13, 1973 [21] Appl. No.1 424,473

Related US. Application Data [63] Continuation-in-part of Ser. No. 283,348, Aug. 24,

1972, abandoned.

[52] US. Cl. 204/40; 75/165; 204/43 G [51] Int. Cl. C25D 3/62 [58] Field of Search 204/43 G, 44, 46 G, 123,

[56] References Cited UNITED STATES PATENTS 2,754,258 7/1956 Taormina et a]. 204/43 G FOREIGN PATENTS OR APPLICATIONS 9,921 3/1971 Japan 204/43 G Primary ExaminerG. L. Kaplan Attorney, Agent, or FirmR. Jonathan Peters 57 ABSTRACT The electroplate produced employing the above electrolytic bath will comprise from about 75 to about 95% gold, from about 5 to about 15% nickel and from about 1 to about 8% zinc and will have a white color characterized by stain and corrosion resistance and by a bright and aesthetically appealing appearance.

18 Claims, No Drawings METHOD OF ELECTROPLATING BRIGHT WHITE GOLD ALLOY COATINGS REFERENCE TO OTHER APPLICATIONS This application is a continuation-in-part of application Ser. No. 283,348, filed Aug. 24, 1972 now abandoned.

The present invention relates to aqueous gold plating solutions and more particularly to aqueous gold plating solutions which include nickel and zinc cyanides, which nickel and zinc codeposit with the gold, to a method of gold plating using such solutions to obtain bright white gold finishes and to gold-plated articles produced thereby.

Gold plating is used extensively for decorative purposes. Color is an important factor in this field. In the gold plating art it is well known to add various alloying metals to a bath in order to provide the desired color gold plate, which may be various shades of green, rose, pink, orange, yellow and white, depending upon the type of article being plated and its intended use. The colors of plated alloys are not the same as thermal alloys and it is difficult to predict the color of binary alloys resulting from a particular plating bath. The difficulty is magnified with plated ternary alloys. A great many baths for plating gold and gold alloys have been proposed. Normally, the baths are tailored to meet the requirements of the particular purpose for which the plated article is used. For example, a gold plate might be satisfactory for jewelry but unsuitable for flatware with respect to either the color, stain or corrosion resistance, or brightness.

Gold plating solutions for use in plating flatware must provide a plate which has a bright whie gold aesthetically appealing appearance while having good stain and corrosion resistance to foods.

The white gold electroplates generally available contain nickel or tin or both as the alloying metal. The plate produced by such baths have not been found suitable for articles such as flatware because of the staining which occurs when the plate is used with foods. This is believed to be caused by the presence of organic acids and sulfides found in edible products. White gold alloys of gold-palladium have been produced by plating, but they are not satisfactory because insufiicient palladium can be codeposited to obtain a suitable white color.

Gold plating baths which contain nickel and other base metals such as zinc are also known. For example, US. Pat. No. 2,724,687 discloses gold plating baths which may contain nickel and other base metals. The base metals in such baths are present only as organometallic compounds and specifically not as cyanides. US. Pat. No. 2,504,601 discloses acid plating baths. The base metals are present as salts of weak organic acids such as formic, acetic, tartaric, and the like, and not as cyanides. None of the proposed baths gives a suitable corrosion resistant white gold. In the latter patent, for example, the specific bath proposed with nickel and zinc contains 8 grams per liter of gold and gives a pale yellow-green coloration.

In accordance with the present invention, an aqueous gold plating solution is provided which produces bright white gold alloy deposits having a pleasing appearance and suitable stain and corrosion resistance properties as well as excellent hardness and wear properties. The gold plating solution of the invention is a stable solution which is easily maintained, and which will provide a constant color and uniform deposition even when the temperature, pH, current density, agitation, metallic contact or anode to cathode ratio vary. The gold alloy deposits produced according to the invention require no buffing or other mechanical operations to further enhance their brightness and have a pleasing color that is acceptable to the flatware trade and are of a sufficiently high Karat to be nontamishing.

The gold plating solution of the present invention comprises an aqueous electrolytic bath having a pH within the range of from about 9 to about 13, and comprises a gold cyanide as the gold source, an alkali metal nickel cyanide as a nickel source, and an alkali metal zinc cyanide as a zinc source, the nickel and zinc codepositing with the gold, an alkali metal cyanide as a stabilizer for the above metal cyanides, at least one conducting salt, and an alkaline material to maintain the bath at the desired pH.

The gold cyanide employed in the aqueous electrolytic bath as the gold source is desirably an alkali metal gold cyanide complex such as potassium aurocyanide or sodium aurocyanide. The gold in the form of the cyanide complex present in the bath ranges from about 0.5 to about 4 grams/liter, and preferably from about 1 to about 2 grams/liter, depending upon the thickness of gold deposit desired. Where gold concentrations of less than 0.5 gram/liter are employed, the gold plate produced will not have adequate staining resistance especially when employed as plate on flatware. Where the gold concentrations employed are above 4 grams/- liter, the plate produced will be too yellow in color and thus will not have the desired white color required in plate for flatware. However, if it is desired to employ the plating solution of the invention in decorating other articles, where a yellow gold plate is desired, the gold cyanide may be employed in quantities greater than 4 grams/liter.

The alkali metal nickel cyanide is preferably employed as the potassium or sodium nickel cyanide. The nickel will codeposit with the gold to provide a hard plate of whitish color and which will have good wear resistance. The nickel in the form of the alkali metal nickel cyanide present in the bath ranges from about 5 to about 15 grams/liter, preferably from about 8 to about 10 grams/liter. Where quantities less than 5 grams/liter of the nickel in the form of the cyanide are employed, the resulting plate will not have the desired corrosion resistance properties required in accordance with the invention. On the other hand, where the amount of nickel in the form of the nickel cyanide present is greater than l5 grams/liter, the staining characteristics of the resulting plate will be adversely affected.

The zinc in the form of the alkali metal zinc cyanide will codeposit with the gold and nickel to impart a whitish color to the plate and enhance the non-staining characteristics of the plate. The zinc is preferably employed in the form of the potassium or sodium zinc cyanide in amounts ranging from about 0.02 to about 0.5 gram/liter, and preferably from about 0.05 to about 0.2 gram/liter. Where the amount of zinc employed is outside the above broad range, the resulting plate will not have the desired whitish color and its nonstaining characteristics will be adversely affected. It is believed that the presence of the zinc even in such low quantities minimizes the tendency of the plate to discolor especially when employed as flatware. It is theorized that this advantageous action of the zinc is at least in part due to the fact that the most probable discoloring agents in food are sulfides which would tend to react with the zinc to produce zinc sulfide which is white in color. Thus, the plate would not show a discoloration.

The plating solution of the invention will also include a free alkali metal cyanide, preferably sodium or potassium cyanide which imparts stability to the solution and especially to the metal cyanides mentioned above. The free alkali metal cyanide present in the bath ranges from about 1 to about 10 grams/liter and preferably from about 3 to about 6 grams/liter. Where amounts less than 1 gram/liter of the alkali metal cyanide is employed, it has been found that the stability of the plating solution will be below that which can be tolerated. No apparent beneficial effects are observed where amounts greater than 10 grams/liter of alkali metal cyanide are employed.

The conducting salt added to the aqueous plating solution is preferably an alkali metal phosphate, such as potassium hydrogen phosphate or sodium hydrogen phosphate. However, other alkali metal conducting salts may be employed such as the alkali metal sulfates, sulfamates, formates, acetates, citrates, lactates, tartrates, fluoborates, borates, other phosphates, carbonates and bicarbonates. The bath may include one, two or more of such conducting salts as long as the added salts are soluble and compatable with all other bath ingredients and will not cause undesired precipitation. Normally, such conducting salts will be employed in an amount within the range of from about 10 to about 100 grams/liter and preferably within the range of from about 20 to about 50 grams/liter. The concentration of conducting salts should be maintained within the above-mentioned broad range inasmuch as if the concentration thereof is below 10 grams/liter, the plating solution will not be sufficiently conductive, while if the concentration thereof is above 100 grams/liter, the specific gravity of the plating solution will be undesirably high with the result that one or more of the metal cyanides may crystallize out of the solution.

An alkaline material is added to the plating solution to maintain the solution within the desired pH range of from about 9 to about 13 and preferably from about 10 to about 12. The alkaline material preferably comprises an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. The alkaline material will ordinarily be employed in an amount within the range of from about 0.5 to about grams/liter to maintain the plating solution at the desired pH. If the plating solution is maintained outside of the above pH range, it will be difficult to obtain the proper codeposition of the metals and the desired color in the resulting plate.

The electroplating process is carried out at a current density within the range of from about to about 100 amperes per square foot and preferably from about 25 to about 75 amperes per square foot, while maintaining the bath at a temperature within the range of from about 40 to about 80C and preferably from about 50 to about 70C. If the current density is maintained below the above-mentioned minimum, an inordinate plating time will be required in order to obtain a plate of desired thickness. On the other hand, if the current density is maintained above 100 amperes per square foot, the resulting plate will be undesirably dull and may be burnt. Furthermore, where the temperature of the plating solution is maintained at below about 40C, the resulting plate will not have the desired brightness,

while if the temperature is maintained above C, this may result in the decomposition of one or more of the components of the plating solution.

The electrodes employed may include such materials as platinum, gold, stainless steel, platinized-titanium or carbon as the anode and such materials as steel, copper. nickel, brass, and the like as the cathode (the wares plated). The ratio of cathode to anode surface area, while not critical, should be within the range of from about 1:1 to about 1:4 and preferably about 1:2. It is customary to agitate the bath, such as with a stirrer, or to move the work (pieces to be plated) as with a cathode oscillating rod, to facilitate uniform and smooth deposition. The voltage between the anode and the wares to be plated as the cathode will usually be between 2 and 6 volts.

My invention provides a heavy plate onto a base metal or ware such as stainless steel, brass, copper, nickel and the like which is bright, stain resistant, corrosion resistant and preferably whitish in color. The plate will have a thickness within the range of from about 5 to about 50 microinches and will provide a gold content of from 19 to 21 Karat. The plate will comprise gold in an amount ranging from about 75 to about 95% and preferably from about 82 to about nickel in an amount ranging from about 5 to about 15%, and preferably from about 8 to about 12%, and zinc in an amount from about 1 to about 8% and preferably from about 2 to about 6%. Such plate will have a Knoop hardness of at least 200 Knoop and in some cases as high as 300 Knoop or higher.

As indicated, the plating solution of the invention is particularly suited for use in plating flatware with a bright white gold coating. It is preferred that in such application, that the base metals be first given a flash coating of bright nickel. Bright nickel flash coatings are well known in the art. Furthermore, the plating solution of the invention can be employed to provide decorative white gold coatings to any desired article such as articles of jewelry, optical frames, watch case besels trays, such as serving trays or ashtrays and the like.

The following Examples represent preferred embodiments of the present invention.

EXAMPLE 1 Aqueous plating baths of the following compositions were prepared:

2 Grams of KOH were added to the baths to adjust the pH thereof to about 10.

EXAMPLE 2 A solution having the composition of BATH A of Example l was heated to 60C. A strip of pure silver 1 by 3 inches was plated at a current density of 70 ASP. A bright white gold which had a slight pleasing yellow tint was deposited. Such plate had a thickness of about 15 microinches and a Knoop hardness of 250.

In order to determine the composition of the plate, the silver base was dissolved out using a cold dilute solution of HNO The resulting white gold plate was analyzed and found to contain:

Gold 82.9% Nickel l l 2% Zinc 5.9%

EXAMPLE 3 A solution having the composition of BATH A was used to plate stainless steel spoons which were first given a bright nickel electrodeposit. The current density used was 70 ASP, the temperature was 60C., and the plating time was three minutes. A bright white gold deposit 15 microinches thick having a Knoop hardness of 250 was obtained. The spoons were tested on a variety of foods over a period of several months. There was no evidence of staining or discoloration of the white gold deposits.

EXAMPLE 4 A solution having the composition of BATH B of Example l was brought to 60C. A strip of pure silver 1 by 3 inches was plated at a current density of 40 ASP. A bright white gold having a pleasing yellow tint was deposited; the resulting plate had a thickness of 18 microinches.

In order to determine the composition of the plate, the silver base was dissolved out using a cold dilute solution of HNO The resulting white gold plate was analyzed and found to contain:

Gold 90% Nickel 8% Zinc 2% EXAMPLE 5 A solution having the composition of BATH B was used to plate stainless steel spoons which were first given a bright nickel electrodeposit. The current density was 40 ASP, the temperature 60C. and the plating time 2.5 minutes. A bright white gold deposit 18 microinches thick was obtained. The spoons were tested on a variety of foods over several months. There was no evidence of staining or discoloration of the white gold deposits.

Various modifications of the invention are contemplated within the scope of the appended claims.

I claim:

1. An aqueous electrolytic bath for gold plating, comprising a gold cyanide as a gold source, said gold cyanide present in said bath providing gold therein in an amount within the range of from about 0.5 to about 4 grams/liter, an alkali metal nickel cyanide, said alkali metal nickel cyanide present in said bath providing nickel therein in an amount within 'the range of from about 5 to about 15 grams/liter, an alkali metal zinc cyanide, said alkali metal zinc cyanide present in said bath providing zinc therein in an amount within the range of from about 0.02 to about 0.5 grams/liter, an alkali metal cyanide, said alkali metal cyanide present in said bath in an amount within the range of from about 1 to about 10 grams/liter, at least one additional conducting salt, and an additional alkaline compound added to said bath to maintain said electrolytic bath at a pH within the range of from about 9 to about 13.

I 2. The electrolytic bath in accordance with claim 1 wherein said gold cyanide is an alkali metal gold cyanide.

3. The electrolytic bath in accordance with claim 2 wherein said alkali metal gold cyanide is potassium aurocyanide.

4. The electrolytic bath in accordance with claim 1 wherein said conducting salt comprises an alkali metal phosphate.

5. The electrolytic bath in accordance with claim 4 wherein said alkali metal phosphate is selected from the group consisting of potassium hydrogen phosphate and sodium hydrogen phosphate.

6. The electrolytic bath in accordance with claim 1 wherein said conducting salt is present in an amount within the range of from about 10 to about grams/- liter.

7. The electrolytic bath in accordance with claim 6 wherein said alkaline compound is present in an amount within the range of from about 0.5 to about 5 grams/liter.

8. The electrolytic bath in accordance with claim 1 wherein said alkali metal nickel cyanide is potassium nickel cyanide.

9. The electrolytic bath in accordance with claim 1 wherein said alkali metal zinc cyanide is potassium zinc cyanide.

10. The electrolytic bath in accordance with claim 1 wherein said alkaline compound comprises an alkali metal hydroxide.

11. The electrolytic bath in accordance with claim 1 wherein the pH thereof is within the range of from about 10 to about 12.

12. The electrolytic bath in accordance with claim 1, wherein gold as said gold cyanide comprises sodium or potassium aurocyanide in an amount within the range of from about 1 to about 2 grams/liter, nickel as said alkali metal nickel cyanide is present in an amount within the range of from about 8 to about 10 grams/- liter, zinc as the alkali metal zinc cyanide is present in an amount within the range of from about 0.05 to about 0.2 gram/liter, the alkali metal cyanide is present in an amount with the range of from about 3 to about 6 grams/liter, the conducting salt comprises an alkali metal hydrogen phosphate in an amount within the range of from about 20 to about 50 grams/liter and the alkaline compound comprises an alkali metal hydroxide in an amount to adjust the pH of said bath to within the range of from about 10 to about l2.

13. A method of electroplating gold alloy onto a material, which comprises electrolyzing an aqueous electrolytic bath comprising a gold cyanide as a gold source, said gold cyanide present in said bath providing gold therein in an amount within the range of from about 0.5 to about 4 grams/liter, an alkali metal nickel cyanide, said alkali metal nickel cyanide present in said bath providing nickel therein in an amount within the range of from about 5 to about 15 grams/liter, an alkali metal zinc cyanide, said alkali metal zinc cyanide present in said bath providing zinc therein in an amount within the range of from about 0.02 to about 0.5 grams/liter, an alkali metal cyanide, said alkali metal cyanide present in said bath in an amount within the range of from about 1 to about 10 grams/liter, at least one additional conducting salt, and an additional alkaline compound added to said bath to maintain said bath at a pH within the range of from about 9 to about 13, to form an electroplated coating of gold alloy comprising gold, nickel and zinc on said material.

14. The method in accordance with claim 13 including the step of applying a coating of bright nickel to the material being plated prior to electroplating said gold alloy on said material.

15. The method in accordance with claim 14 wherein the material being plated comprises metal flatware.

16. The method in accordance with claim 13 wherein said electroplated coating of gold alloy comprises from about to about gold, from about 5 to about 15% nickel and from about 1 to about 8% zinc.

17. The method in accordance with claim 13 wherein said electroplated coating of gold alloy comprises from about 82 to about 90% gold, from about 8 to about 12% nickel and from about 2 to about 6% zinc.

18. The method in accordance with claim 13 wherein said aqueous electrolytic bath contains said conducting salt in an amount within the range of from about 10 to about grams/liter and said alkaline compound in an amount within the range of from about 0.5 to about 5 grams/liter. 

1. AN AQUEOUS ELECTROLYTIC BATH FOR GOLD PLATING, COMPRISING A GOLD CYNADIDE AS A GOLD SOURCE, SAID GOLD CYNIDIDE PRESENT IN SAID BATH PROVIDING GOLD THREIN IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 0 .5 TO ABOUT 4 GRAMS/LITER, AN ALKALI METAL NICKE CYNIDIDE, SAID ALKALI METAL NICKEL CYNADIDE PRESENT IN SAI BATH PROVIDING NICKEL THEREIN IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 5 TO ABOUT 15 GRAMS/LITER, AN ALKALI METAL ZINC CYANIDE, SAID ALKALI METAL ZINC CYANIDE PRESENT IN SAID BATH PROVIDING ZINC THEREIN IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 0.02 TO ABOUT 0.5 GRAMS/LITER, AN ALKALI METAL CYANIDE, SAID ALKALI METAL CYANIDE PRESENT IN SAID BATH IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 1 TO ABOUT 10 GRAMS/LITER, AT LEAST ONE ADDITIONAL CONDUCTING SALT, AND IN ADDITIONAL ALKALINE COMPOUND ADDED TO SAID BATH TO MAINTAIN SAID ELECTROLYTIC BATH AT A PH WITHIN THE RANGE OF FROM ABOUT 9 TO ABOUT
 13. 2. The electrolytic bath in accordance with claim 1 wherein said gold cyanide is an alkali metal gold cyanide.
 3. The electrolytic bath in accordance with claim 2 wherein said alkali metal gold cyanide is potassium aurocyanide.
 4. The electrolytic bath in accordance with claim 1 wherein said conducting salt comprises an alkali metal phosphate.
 5. The electrolytic bath in accordance with claim 4 wherein said alkali metal phosphate is selected from the group consisting of potassium hydrogen phosphate and sodium hydrogen phosphate.
 6. The electrolytic bath in accordance with claim 1 wherein said conducting salt is present in an amount within the range of from about 10 to about 100 grams/liter.
 7. The electrolytic bath in accordance with claim 6 wherein said alkaline compound is present in an amount within the range of from about 0.5 to about 5 grams/liter.
 8. The electrolytic bath in accordance with claim 1 wherein said alkali metal nickel cyanide is potassium nickel cyanide.
 9. The electrolytic bath in accordance with claim 1 wherein said alkali metal zinc cyanide is potassium zinc cyanide.
 10. The electrolytic bath in accordance with claim 1 wherein said alkaline compound comprises an alkali metal hydroxide.
 11. The electrolytic bath in accordance with claim 1 wherein the pH thereof is within the range of from about 10 to about
 12. 12. The electrolytic bath in accordaNce with claim 1, wherein gold as said gold cyanide comprises sodium or potassium aurocyanide in an amount within the range of from about 1 to about 2 grams/liter, nickel as said alkali metal nickel cyanide is present in an amount within the range of from about 8 to about 10 grams/liter, zinc as the alkali metal zinc cyanide is present in an amount within the range of from about 0.05 to about 0.2 gram/liter, the alkali metal cyanide is present in an amount with the range of from about 3 to about 6 grams/liter, the conducting salt comprises an alkali metal hydrogen phosphate in an amount within the range of from about 20 to about 50 grams/liter and the alkaline compound comprises an alkali metal hydroxide in an amount to adjust the pH of said bath to within the range of from about 10 to about
 12. 13. A METHOD OF ELECTROPLATING GOLD ALLOY ONTO A MATERIAL, WHICH COMPRISES ELECTROLYZING AN AQUEOUS ELECTROLYTIC BATH COMPRISING A GOLD CYANIDE AS A GOLD SOURCE, SAID GOLD CYANIDE PRESENT IN SAID BATH PPROVIDING GOLD THEREIN IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 0.5 TO ABOUT 4 GRAMS/LITER, AN ALKALI METAL NICKEL CYANIDE, SAID ALKALI METAL NICKEL CYANIDE PRESENT IN SAID BATH PROVIDING NICKEL THERIN IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 5 TO ABOUT 15 GRAMS/LITER, AN ALKALI METAL ZINC CYANIDE, SAID ALKALI METAL ZINC CYANIDE PRESENT IN SAID BATH PROVIDING ZINC THEREIN IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 0.02 TO ABOUT 0.5 GRAMS/LITER, AN ALKALI METAL CYANIDE, SAID ALKALI METAL CYANIDE PRESENT IN SAID BATH IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 1 TO ABOUT 10 GRAMS/LITER, AT LEAST ONE ADDITIONAL CONDUCTING SALT, AND AN ADDITIONAL ALKALINE COMPOUND ADDED TO SAID BATH TO MAINTAIN SAID BATH AT A PH WITHIN THE RANGE OF FROM ABOUT 9 TO ABOUT 13, TO FORM AN ELECTROPLATED COATING OF GOLD ALLOY COMPRISING GOLD, NICKEL AND ZINC ON SAID MATERIAL.
 14. THE METHOD IN ACCORDANCE WITH CLAIM 13 INCLUDING THE STEP OF APPLYING A COATING OF BRIGHT NICKEL TO THE MATERIAL BEING PLATED PRIOR TO ELECTROPLATING SAID GOLD ALLOY ON SAID MATERIAL.
 15. The method in accordance with claim 14 wherein the material being plated comprises metal flatware.
 16. The method in accordance with claim 13 wherein said electroplated coating of gold alloy comprises from about 75 to about 95% gold, from about 5 to about 15% nickel and from about 1 to about 8% zinc.
 17. The method in accordance with claim 13 wherein said electroplated coating of gold alloy comprises from about 82 to about 90% gold, from about 8 to about 12% nickel and from about 2 to about 6% zinc.
 18. The method in accordance with claim 13 wherein said aqueous electrolytic bath contains said conducting salt in an amount within the range of from about 10 to about 100 grams/liter and said alkaline compound in an amount within the range of from about 0.5 to about 5 grams/liter. 